1. Field of the Invention
The present invention generally relates to adaptive control, and more particularly, to an adaptive controller and an adaptive control method for adaptively controlling at high speed a plurality of variable high frequency devices (hereinafter, the “variable high frequency devices” refers to high frequency devices whose impedances are variable).
2. Description of the Related Art
In the above-mentioned kind of field, based on a signal varied depending on the impedances of a plurality of high frequency devices, each impedance is adaptively controlled to an optimum value. In a communication environment varied with time, for example, good wireless communication is performed by adaptively controlling impedances of high frequency devices such as capacitors and inductors associated with a plurality of antenna elements.
FIG. 1 shows such an adaptive control system. In this example, each of M antenna elements 102 is connected to a high frequency device 104. High frequency devices 104 are capable of varying the respective impedances z1 through z2M in accordance with control signals x1 through x2M. The M antenna elements 102 are connected to a combiner 106. The output of the combiner 106 is supplied to an adaptive controller 108. Signals received by the respective antenna elements 102 are combined under influences of the impedances z1 through z2M of the respective high frequency devices 104 and are introduced in the adaptive controller 108. In the adaptive controller 108, by using an appropriate optimizing algorithm, the control signals x1 through x2M are created such that a signal from the combiner 106 has a desired characteristic, thereby varying the respective impedances. Hence, it becomes possible to suppress only interference waves and process only desired waves in the subsequent stages among a plurality of arriving waves that are received by the antenna elements 102.
FIG. 2 shows a conventional control flow 200 that can be used in an adaptive controller. The method is an optimizing method using a perturbation method. The control flow 200 starts in step 202. In step S204, impedances zi of a plurality of elements (inductors Li and capacitors Ci) or control signals xi for setting them are set to appropriate initial values. In step S206, the value of a parameter m for indicating an element that is subject to control is set to an initial value 0.
In step S208, the value of an evaluation function f(z) varied depending on an output signal y(z) (Z=(z1, z2, . . . z2M)) from the combiner 106 is calculated. The value of the evaluation function f(z) is stored in a memory as a reference value fo(z). Various functions may be adopted as the evaluation function f(z). It is possible, for example, to adopt a function representing the coincidence of received signals y and known signals (preambles) r that are inserted into a transmission signal at regular intervals as follows.ƒ=|yHr|/(√{square root over (y*y)}√{square root over (r*r)})In this case, H represents obtaining an Hermitian conjugate (to obtain a complex conjugate by transposing a vector).
The above equation is equivalent to calculation of an inner product or a scalar product of a standardized received signal y and a standardized known signal r. Accordingly, the value of the function f(z) is a real value having the absolute value of 1 or less: when the received signal y and the known signal r match, the value of the function f(z) is 1, and when the received signal y and the known signal r are orthogonal, the value of the function f(z) is 0.
In step 210, the value of the parameter m is incremented by 1.
In step 212, the value of impedance zm of the “m”th element is varied to zm+Δzm. When the value of “m” is 1, for example, the value of an inductor L1 is slightly varied. The slight variation Δzm in the impedance zm results in variation in the output signal y(z).
In step 214, the value of the evaluation function E f(z) is calculated by using the frame next to the frame used in the calculation in step 208.
In step 216, a gradient vector ∇f is calculated by calculating the difference between the values of the evaluation function f(z) before and after the variation with respect to the impedance zm. ∇f is a vector quantity having 2M components, and each component is calculated by the following equation.                                           (                          ∇              f                        )                    zm                ⁢                =                              f            ⁡                          (                                                z                  1                                ,                …                ⁢                                                                  ,                                                      z                    m                                    +                                      Δ                    ⁢                                                                                  ⁢                                          z                      m                                                                      ,                …                ⁢                                                                  ,                                  z                                      2                    ⁢                    M                                                              )                                -                                                     ⁢                  fo          ⁡                      (                                          z                1                            ,              …              ⁢                                                          ,                              z                m                            ,              …              ⁢                                                          ,                              z                                  2                  ⁢                  M                                                      )                              
In step 218, the value of the slightly varied zm is returned to the value before the slight variation.
In step 220, it is determined whether the value of the parameter m is 2M or less. When the value of the parameter m is 2M or less (YES in step 220), the flow returns to step 208, and another component of the gradient vector ∇f is calculated. On the other hand, when the value of the parameter m is not 2M or less (NO in step 220), i.e., greater than 2M, which means all components of the gradient vector ∇f are calculated, the flow proceeds to step 222.
In step 222, the value of the impedance zi of each element is updated by using the gradient vector ∇f. The gradient vector ∇f indicates the direction in which the gradient (inclination) is most drastically varied from a coordinate (z1, z2, . . . , z2M) on an f surface. Accordingly, it is possible to approach the maximum or minimum value (a desired optimum value) of the evaluation function f(z) by advancing in the direction indicated by ∇f from the coordinate. A parameter α represents the step width in advancing along ∇f when updating the value of the impedance zi. In the aforementioned manner, the value of the impedance zi is updated.
In step 224, it is determined whether sufficient convergence is obtained by comparing the value of the previous impedance with that of the updated impedance. When sufficient convergence is not obtained (NO in step 224), the flow returns to step 206. When sufficient convergence is obtained (YES in 224), the flow proceeds to step 226 where the control flow 200 ends.
Methods for optimizing impedance by calculating the gradient vector ∇f by using a perturbation method and sequentially updating the impedance as mentioned above are described in, for example, Japanese Laid-Open Patent Application No. 2002-118414, and Jun Cheng, Yukihiro Kamiya, and Takashi Ohira, “Adaptive Beamforming of ESPAR Antenna Based on Steepest Gradient Algorithm”, IEICE TRANS. COMMUN., VOL.E84-B, No. 7, Jul. 2001.
The high frequency devices 104, which are used in conventional adaptive control systems, are generally formed by semiconductor devices such as, varactor diodes in light of high-speed operations. It is possible to cause such a kind of semiconductor devices to perform a vary high speed operation, i.e., to vary the impedance in a very short time interval, for example, in 10−12 seconds. However, such semiconductor devices have a problem in that comparatively a great deal of electric power is consumed. In addition, such semiconductor devices have problems in that insertion loss may be high, isolation characteristics may be low, and cost may be high, for example. Accordingly, it is not easy to perform adaptive control as mentioned above by mounting such a kind of semiconductor devices on a small electronic device.
On the other hand, with the progress in silicon processing technologies, technologies referred to as Micro Electro Mechanical System (MEMS) or Nano Electro Mechanical System (Nano EMS) are attracting attention these days. An Electro Mechanical System (hereinafter referred to as an “EMS”) in the order of micrometers or nanometers is a minute mechanical system having a size of approximately micrometers or nanometers. It is possible to build a variable high frequency device using an EMS by, for example, mechanically varying the distance between the polar plates of a capacitor, or by varying the insertion amount of a magnetic core of an inductor. Variable high frequency devices or variable impedance devices using EMSs are advantageous in isolation characteristics, insertion loss, costs, and the like, as-well as in having an electric power consumption less than that of semiconductor devices, which mainly results from mechanical operations thereof.
However, it is necessary for variable high frequency devices using EMSs to move moving parts thereof so as to vary impedance. For this reason, such variable high frequency devices are disadvantageous in that the working speed thereof is slower than that of semiconductor devices. Suppose the high frequency device 104 is formed by four elements each using an EMS, and each of the elements requires 100 μs to vary the impedance, for example. In this case, each of the elements requires 200 μs for varying the impedance twice in steps 212 and 222. Consequently, the update of impedance in step 222 is performed only once in (twice×4 elements+1 (for updating))×100 μs=900 μs.
On the other hand, suppose the high frequency element 104 is formed by four elements each using an EMS and being capable of varying the impedance at a comparatively high speed such as 4 μs, though which speed is slower than that of semiconductor devices. In this case, in theory, it is possible to perform the update of impedance in step 222 once in (twice×4 elements+1 (for updating))×4 μs=36 μs. However, the calculations of the value of the evaluation function in steps 208 and 214 use the known signals included in frames. Thus, it is not always possible to update the impedance at high speed as such. Even in a high speed wireless LAN standard such as. IEEE802.11a, for example, merely 8 μs preamble is obtained for each frame of 20 μs. Accordingly, in the above-mentioned case, the update of impedance in step 222 is performed only once in (twice×4 elements+1 (for updating))×20 μs=180 μs (9 frames).